Myths and Truths about PCI LATENCY

Hello all,

From reading different threads on different forums I've stumbled across PCI Latency and learned more about it.

I've read that setting the PCI Latency in BIOS has NO effect. Windows has the power to override it.

What is PCI LATENCY? I read that it is the amount of time a certin device is given the attention by the cpu. Setting it higher will allow it more time with cpu, but the drawback is, other device will have to wait their turn. When one device is set higher than others...e.g. video 248ms, sound at 32ms, the video card is getting a significantly more time than the sound card. Thus, causing sound problems.

I've read that ATI cards are default set to 248ms. (my speculation as well as others are that they want to eliminate any problems with video for themselves. But this is quite selfish and irresponsible to hog the pci bus this way)

Myths. Lowering the PCI Latency does not MAKE that device FASTER. Its not the same as setting MEMORY RESPONSE TIME. Setting the PCI LATENCY lower only means it gets less time with cpu processing.

Fact: So there needs to be a balance of pci latencies between different devices. Setting them to be more equal will allow a more equal processing time. I read setting it to 64 is a magic number.

Fact: AGP is a desiginated slot OF PCI for Video Cards. PCI Latency DOES affect agp.

(please move this thread if you feel that there is a more appropriate section. I didn't know where to put it, and thought video card latency is set too high)

 

So the question I have now is....

How can we change PCI Latency manually in windows? (since bios does not do anything).

I've read about people using "powerstrip". Does this program HAVE to be running on background to keep pci latency to what you want? Or can you delete it after you have set it?

Is there any other way to change the PCI Latency... in registry? Any other good programs? Does NVIDIA System Utility do this (not talking about the unified drivers, but the other nForce driver on nvidia website?)

I wanted to post this thread to inform others truths and myths of pci latency. So if there is anything you like to add on, or correct my mistakes, please feel free to do so. I want to learn more about it myself too.

 

search your registry for "DisableProgPCILatency" and change the value from 0 to 1 that will stop the ati drivers from setting your vid card latency to 248ms

 

any way to monitor the latency values?

setting value from 0 to 1. what will it become? Whatever u set it in bios?

 

use this to see what they are set to link

and yes if u do that in the reg it will set it back to what ever u have set it 2 in the bios

before

and after

 

Hello Heat-40, so yeah, thanks for your advice. I set the registry to "1", and it seems to correct my sound problem. I have a couple of questions.

1. Where did u get that pci latency tool?
2. Does that tool allow you to set the values? or view only?
3. Where can I find a program that will let me adjust the value as I see fit?

 

I thought I read somewhere that for users of PCI-Express graphic cards that they don't have to worry about sharing PCI bandwidth with other PCI cards because they have dedicated bandwidth. Is this true?

 

I've got this weird sound problem in some games like there is this weird chirpy noise but only when I look in a certain direction? will changing this pci latency thing fix that, do you think?
I'm not even sure what to set it to in the bios if it would fix it.

 

Hmmm could it be a bird in the trees, that u can only hear, when u face towards it? LOL.

if its not a bird. Have you tried updating chipset driver? sound driver?

if yes to both and still a prob, then i finally resort to setting the pci latency to either 32 or 64 in bios, then remember to edit the registry. (be careful, screwing up registry = BOOOM!) like heat-40 said, search for DisableProgPCILatency, with Edit Menu/Find. and change the value to 1.

If you have some other programs that can edit the pci latency, I read others saying 64 for video and 32 for sound. But most of them say just change it, test it and see. (trial and error). good luck hope it helps.

 

hi all, i just found this thread thru the PCI Latency Tool v2.0 thread. question is, i got a ton of DisableProgPCILatency values in my registry

which ones do i set to 1 ? any particulat one ? or all of them ?

 

download the latest version of ati tool,it has the ability to overwrite the pci latency of you video card.

 

if set right the pci latency tool 2.0 can be used without modifiying the registry as it loads after windows changing it to whatever you set it to

alot of people just aren't setting it right

 

johndoe, i don't know why u are getting so many returns. Where are they located?

HLH, no offense, but i don't quite get what you mean.

NETCORE, i will try pci latency tool myself also. Although editting the registry helped, i still get static in rare occasions. Sounds like most people benefit and approve of this tool, i will give it a shot.

 

So what should you set it to.. or is it one of those 'suck and see' approaches..?

 

i started at 128 and am now at 64... seems less stuttery at 64

 

what should u set what to? registry value? set to 1. pci latency, set agp to 64, and others such as sound to 32. but try around and see.

 

i dl that pci latency tool, all my devices show 64 without me having done anything. guess im ok then.


http://craigwp.customer.netspace.net.au/LtcyCfg.zip

 

DisableProgPCILatency found at following locations:

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0000\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0001\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Video\{0C5AF3E1-BADC-413D-A6E5-3D1891E4B358}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Video\{C9F7B253-F2FB-4A2D-AE52-20544B383D6B}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\ati2mtag\Device0]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\ati2mtag\Device1]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\ati2mtag\Device2]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\ati2mtag\Device3]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0000\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0001\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Video\{0C5AF3E1-BADC-413D-A6E5-3D1891E4B358}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Video\{C9F7B253-F2FB-4A2D-AE52-20544B383D6B}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Services\ati2mtag\Device0]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Services\ati2mtag\Device1]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Services\ati2mtag\Device2]

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Services\ati2mtag\Device3]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0000\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}\0001\Settings]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\{0C5AF3E1-BADC-413D-A6E5-3D1891E4B358}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\{C9F7B253-F2FB-4A2D-AE52-20544B383D6B}\0000]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ati2mtag\Device0]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ati2mtag\Device1]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ati2mtag\Device2]

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ati2mtag\Device3]

 

I was looking to go the regedit way cuz I wanted the settings to stick without using a third party program if ya know what I mean

 

Yes. Here's a brief breakdown of how it works. The regular PCI bus is like a hub, multiple connection sharing the bus, but only one device can actually talk at a time. Changing the latency timing determines how long each "port" or slot can use the bus before giving up its turn to another device. A true PCI-Express (not to be confused with PCI-X) bus is more like a switch where multiple "conversations" can be had at the same time. PCI-E buses have 32 lanes available to them at 250mhz (or mbs, I don't recall offhand). These lanes can be combined to allow a single slot multiple lanes. That's why you see mother boards with 16x, 8x, 4x and 1x slots. Most of the PCI-E video cards, at least the high-end ones, are 16x. This means that they can use 16 X 250 =4gbs memory throughput, whereas AGP is limited to 1.2ghz (me thinks). Things like your sounds card will only need the 1x slot. That's why those lucky enough to do so, have to be careful in purchasing an SLI mobo. A lot of them have only 1 16x slot and then the second one is 8x. Then again, two 16x PCI-E slots, would use the entire PCI-E bus. Maybe you'll have mobos with 2, I don't know. Most of the current mobos that are PCI-E also include regular PCI buses for your other devices. Someday you will be able, for example, to buy a PCI-E sound card.

Does that help?